More than 3 million people a year are killed prematurely by outdoor air pollution, according to a landmark new study, more than malaria and HIV/Aids combined.

Wood and coal burning for heating homes and cooking is the biggest cause, especially in Asia, but the research reveals a remarkably heavy toll from farming emissions in Europe and the US, where it is the leading cause of deaths.

But the research found that agricultural emissions of ammonia had a “remarkable” impact, according to Professor Jos Lelieveld, at the Max Planck Institute for Chemistry in Germany, who led the research. A fifth of all global deaths resulted from these emissions, which come mainly from cattle, chickens and pigs and from the over-use of fertiliser.

The ammonia reacts with fumes from traffic and industry to produce tiny particles and is the largest cause of air pollution deaths in the eastern US, Japan and in Europe. “For London, agriculture is the main source,” said Lelieveld. Across the UK, 48% of the premature deaths were ultimately the result of agricultural pollution.

Currently it is assumed that all particulates are equally toxic. But if those from fuel burning are more toxic, as some scientists suspect, the proportion of premature deaths attributable to farming emissions would be lower. Even so, Professor Michael Jerrett, environmental health expert at the University of California said “the finding is highly valuable … because agriculture has generally not been seen as a major source of air pollution or premature death, and because it suggests that much more attention needs to be paid to agricultural sources, by both scientists and policymakers.”

Globally, more than 3.2 million premature deaths per year are attributed to exposure to ambient fine particulate matter (PM2.5). A new study estimates that 2.1 million premature deaths could be avoided if countries achieved the WHO guideline for PM2.5. Even meeting their closest WHO interim concentration targets could avoid 750 000 (23%) deaths attributed to PM2.5 per year.

The World Health Organization has issued a specific guideline value for ambient PM2.5, set at 10 ug/m3 as an annual average. It has further given interim targets concentrations for PM2.5 at 35, 25, 15 ug/m3, target values that can be used by governments to approach the final guideline value of 10 ug/m3 in a stepwise manner.

The study found that limiting PM2.5 concentrations to WHO interim target levels 35, 25, 15, or 10 µg/m3, could avoid approximately 0.39, 0.73, 1.4, and 2.1 million annual premature deaths, respectively. Another strategy, called ‘next target’ (a country at 27 µg/m3 would try to get down to its closest lower target of 25 µg/m3, for example) could save approximately 750 000 lives globally.

The study also revealed the impact of demographic factors and the (non-linear) exposure-response function on PM2.5 mortality. Highly polluted areas require a greater reduction of PM2.5 than less polluted areas to achieve the same decrease in attributed mortality. In the most highly polluted areas, such as India and China, a 50% reduction in mortality would require a 68% reduction of PM2.5. Conversely, a 50% mortality reduction in less polluted areas, such as Europe and North America, could be achieved through a reduction of just 25% of PM2.5.

The researchers stress that initial improvements to ambient PM2.5 concentrations must be worked into long-term mitigation strategies, which will differ greatly between more or less polluted areas. Population age structure should also be taken into account when planning mitigation policies, they say. In fact, if the world met the World Health Organization 10 µg/m3 target, nearly 70% of the total mortality avoided would be concentrated in East Asia and India because these areas are densely populated and currently experience high PM levels. Importantly, the researchers say there is also high potential for reducing mortality in less polluted areas as well. Furthermore, there could also be positive impacts on climate change because PM2.5 mitigation may also reduce the emission of other accompanying pollutants, such as black carbon.